Clemens in U.S. Pat. Nos. 3,842,194, 3,842,217 and 3,909,517 has described a capacitive information disc record comprising a molded plastic disc record having audio and video information in the form of geometric variations in a spiral groove in the disc record surface. These disc records are conformally overcoated with a conductive material, which acts as a first electrode, and then further overcoated with a dielectric layer. A metal-tipped stylus acts as the second electrode of a capacitor and the information signals recovered as variations in capacitance between the stylus electrode and the disc record conductive layer are processed to reconstitute the audio and video information. Relative motion is established between the disc record and the stylus during information recovery.
Capacitive information disc records have also been developed which do not require a grooved surface. With this type of disc record the stylus is maintained in synchronization with an information pattern in the disc record surface by means of electrical signals recorded on either side of the information track rather than physically by means of the groove walls.
Fox et al., in a copending application entitled "A Conductive Video Disc," Ser. No. 105,550, filed Dec. 20, 1979, disclose a capacitive information disc record which is made of a conductive molding composition containing a molding resin such as poly(vinyl chloride) and a sufficient amount of conductive particles such as carbon black so that the disc record can provide capacitance readout without a conductive coating. The conductive molding composition may also contain additives such as flow modifiers, lubricants, stabilizers and the like. This development has eliminated the need for separate coatings of metal and dielectric layers on the disc record and may be used for either grooved or nongrooved capacitive information disc records.
Carbon blacks suitable for use in a capacitive information disc record conductive molding composition include highly electrically-conductive, finely-divided carbon blacks preferably having a low bulk density to reduce loading requirements. These carbon blacks should have a high surface area and a high proportion of voids within the particles which enables current to flow between the conductive particles.
The amount of carbon black present in the conductive molding composition depends on the electrical properties required. For example, the amount of carbon black is generally directly related to the conductivity of the resultant information disc record. However, large quantities of fillers such as carbon black decrease the processability of a molding composition and increases the brittleness of the product. Fox et al. in the aforementioned copending application have found that Ketjenblack EC carbon black (a product of Akzo Chemie) present in amounts from about 10 to about 20 percent by weight achieves the desired molding composition conductivity and processability.
It has been found that batch-to-batch variations occur in the properties of the carbon black which affects the ability to reliably mass produce the capacitive information disc records. These variations affect the performance properties of the capacitive information disc record. Performance properties are those properties which relate to the quality of the information signal which is recorded in the capacitive information disc record and include the video carrier signal-to-noise ratio (VCN) and audio carrier signal-to-noise ratio (ACN). Even when the same carbon black product is employed at the same weight percent loading in the same molding composition formulation, batch-to-batch variations in the performance properties of the capacitive information disc records occur.
Furthermore, when an optimum composition for obtaining certain performance properties has been found, which contains a determined amount of carbon black from a given batch, attempts to prepare additional amounts of the optimum composition from the same carbon black batch by mixing compositions having higher and lower amounts of carbon black to obtain the same average amount of carbon black were unsuccessful. The mixing of compositions did not result in capacitive information disc records having the same performance properties as those prepared from the original optimal composition.
Therefore, it is desirable to develop a method for preparing conductive molding compositions in order to produce capacitive information disc records having uniform and predeterminable performance properties.